634 THE PLEISTOCENE PERIOD 



Iowa is an irregular sheet of sand and gravel with remnants of old 

 soil, muck, and peat, with stumps and branches of trees. The sur- 

 face of the drift beneath shows much weathering and erosion. The 

 fossils in these interglacial beds imply a cool- temperate climate; 

 but as a cool-temperate stage must be passed through twice between 

 successive glacial epochs, once as the ice retreats, and a second 

 time as it advances again, fossils indicating a cool climate do not 

 necessarily show how warm the interglacial epoch may have become. 



III. Kansan glacial stage. The Kansan stage is represented by 

 a sheet of till occupying a large surface area in Kansas, Missouri, 

 Iowa, and Nebraska. Theoretically it extends under the later gla- 

 cial formations to the northward, as far back as the Keewatin 

 center of radiation. Much of this sheet of drift, as originally devel- 

 oped, probably was rubbed away by later glaciations. Presumably 

 a similar sheet was formed by a contemporaneous ice-sheet spread- 

 ing from the Labrador center, but it has not been certainly identi- 

 fied. The Kansan till is clayey and there is little stratified drift 

 associated with it. 



IV. The Yarmouth interglacial stage. 1 Where the Illinois till 

 overlaps the Kansan (eastern Iowa), an old soil, with deep subsoil 

 weathering, lies on the surface of the latter. 



V. Illinoian and lowan glacial stages or lowa-Illinoian stage. 

 On the borders of the Labradorean field near the Mississippi River, 

 the Illinoian drift sheet overlies the Kansan sheet with the Yarmouth 

 beds between. In the Keewatin field in eastern Iowa, the lowan 

 drift lies over the Kansan, with the Buchanan beds between. 

 Some geologists now think that the lowan represents the same stage 

 in the Keewatin field that the Illinoian does in the Labradorean 

 field; i. e., the third ice invasion. The earlier view was that the 

 Illinoian drift was the older. 



V. A. Illinoian drift sheet (Labradorean field). The exposed por- 

 tion of this drift occupies the surface in the southern and western 

 parts of Illinois. It runs back under the later drift to the north- 

 east toward the Labradorean center. To the eastward, it is traced 

 as far as Ohio, where it is covered by later drift. To the northward 

 its margin is covered in southern Wisconsin, but in central Wis- 

 consin it seems to re-appear and is traced westward on the north 

 side of the driftless area, beyond which, m Minnesota, it seems to 

 connect with the lowan drift. The Illinoian till is clayey, with little 



1 Leverett, Mono. XXXVIII, U. S. Geol. Surv. 



